Glass heating fabric

ABSTRACT

A heater fabric and method of fabrication are provided, wherein the fabric heater comprises a woven glass fabric having a layer in contact with electrodes of a conductive mixture comprising a vinylidene chloride polymer and carbon to provide the desired watt density and an outer coating of a vapor barrier of a vinylidene chloride polymer. A fabric softener layer may also be provided between the glass fabric and the conductive coating to impart greater flexibility to the heater fabric.

United States Patent [191 Barnes et a1.

[ GLASS HEATING FABRIC [75] inventors: Richard D. Barnes, Stokesdale,

Rockingham County; William E. Sharpe, Jr., Greensboro, both of NC.

[73] Assignee: Burlington lndustries, 1nc.,

Greensboro, NC.

221 Filed: Feb. 12,1974

211 App|.No.:441,817

[52] US. Cl. 338/211; 219/528; 219/543;

219/549; 252/511; 338/212 [51] Int. Cl. 1105b 3/36 [58] Field of Search 219/211, 212, 217, 527,

[56] References Cited UNITED STATES PATENTS 2,473,183 6/1949 Watson .1 219/543 2,559,077 7/1951 Johnson et a1 219/543 2.991.257 7/1961 Smith-Johannson 252/506 [451 Apr. 8, 1975 3.056.750 10/1962 Pass 252/511 3,287,684 11/1966 Armbruster, Jr. 338/211 3.385.959 5/1968 Ames et a1 219/549 3.387.248 6/1968 Rees 338/211 3.400.254 9/1968 Takemori 219/549 3.808.403 4/1974 Kanaya et a1. 219/528 Primary E.\'aminer-v010dymyr Y. Mayewsky Attorney, Agent, or F irmCushman, Darby & Cushman [57] ABSTRACT A heater fabric and method of fabrication are provided, wherein the fabric heater comprises a woven glass fabric having a layer in contact with electrodes of a conductive mixture comprising a vinylidene chloride polymer and carbon to provide the desired watt density and an outer coating of a vapor barrier of a vinylidene chloride polymer. A fabric softener layer may also be provided between the glass fabric and the conductive coating to impart greater flexibility to the heater fabric.

8 Claims, 8 Drawing Figures fis/srm'ca Owns/6204.6! 50/- ya/I i GLASS HEATING FABRIC BACKGROUND OF THE INVENTION The present invention relates to a heater fabric and method of making the same.

For many years the only satisfactory source ofa semiflexible heater material was a product known as a heater cable. By arranging these heater cables in equally spaced configurations it was possible to provide a large flat area of radiantheat source. These heater cables were all made by insulating an alloy resistance wire, such as Chromel (Trademark of Haskins Mfg. Co.

for a series of Ni-Cr alloys) and then encasing the insulated heater wire in a metallic sheath.

More recently, there have been many attempts to develop other types of flexible heater products. such as woven graphite fibers, sheets of conducting material sandwiched between a film or fabric coated with conducting compounds and enclosing ininsulating filrn. See US. Pat. Nos. 3,l46,340 and 3,400,254.

In all cases, when these products were put in service it was necessary to encase them in protective film to protect them from moisture and from the hazardous contact with the surface of the live electric circuit. However, the presence of this film is a serious deterrent to its use in'many applications, such as molded ceiling tile, plaster board, etc.

U.S. Pat. No. 2,473,183 teaches coating a woven fabric with a vinyl resin containing conducting carbon black, plus an over coating of a non-conductive material, such as plasticized vinyl resin to protect a person from getting electrical shock when touching the surface.

The insulating coating of this patent being a plasticized vinyl resin will not sufficiently protect the conducting coating since a 3 mil vinyl coating would have a moisture vapor transmission of from 0.5 to 1.0 perms.

U.S.Pat. No. 3,112,985 teaches the bonding ofa film of polyvinylidene chloride to paper for the purpose of making it greaseproof and resistant to the passage therethrough of water vapor.

US. Pat. No. 3,359,525 teaches coating glass fabric with a conductive coating of polyimide resin and conducting carbon black followed by an insulating coating of a polyimide resin. This provides an insulating coating over the conductive coating, but not a good vapor varrier. This product is designed to operate at 600F.

None of these products has proven satisfactory, as witnessed "by the fact that the Underwriters Laboratories, Inc. do not include any such heater devices on its list of approved radiant heat sources.

The heater cable is still the only recognized and approved source of radiant heat for installation in homes, driveways, etc.

It is the object of this invention to produce a flexible heat source that will satisfactorily overcome all the previous objections and provide a reliable heat source in the range of to 70 watts per square foot. Such a product would be suitable for a large variety of end uses, such as home heating in the formof a ceiling tile or ceiling board, preventing ice on bridges, roadways, and airport runways, heated plant beds, matress pads, heated storage tanks, etc.

SUMMARY OF THE INVENTION A heater fabric, and a method of fabrication are provided wherein the heater fabric comprises a woven glass fabric coated with a layer of a conductive mixture comprising a vinylidene chloride polymer and carbon to provide the desired watt density and an over coating of a vapor barrier of a vinylidene chloride polymer. A fabric softener layer may be provided between the glass fabric and the conductive coating to impart greater flexibility to the heater fabric.

The vapor barrier of the vinylidene chloride polymer protects the conducting layer from moisture and accidental contact with the live circuit.

Unexpectedly it has been found that the vapor barrier coating serves further to bond the conducting particles in the base coat more compactly together giving the product a more uniform resistance. The vapor barrier coating also improves the flex properties of the heater fabric.

Another unexpected feature of the glass heater fabric of the present invention is that cutouts, such as needed for a lamp fixture, can be made without affecting its ability to continue to operate as a heat source.

An advantage of the present invention is the ease with which the heater fabric can be incorporated in an article of manufacture, as for example, a plaster board.

Plaster boards with a heater cable as an integral part of the board for radiant ceiling heating are known. This product is made for example by routing out a trench in the plaster board, inserting an insulated heater cable, filling the trench with plaster. and facing the board with paper.

The glass heater fabric which will be used according to the present invention for incorporation into such articles as plaster board, ceiling tile, etc. is an open weave construction having openings approximately 1/16 inch square. Because each individual yarn in the fabric is surrounded by the vapor barrier coating and is insulated and protected from moisture, the glass heater fabric can be molded in the wet plaster slurry in a simple operation. The wet plaster slurry will key through the openings in the heater fabric and the heater fabric will become an integral part of the structure. Not only will the glass heater fabric serve as a heat source, but it will also reinforce the plaster board. In contrast, with a conventional heater fabric enclosed in a plastic film, the plaster will not stick to the film, and subsequent delamination will occur.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a semi-log graph of the resistance of a heater fabric versus the carbon/PVDC ratio of the conducting layer in the heater fabric;

FIG. 2 is a schematic cross section of a heater fabric according to the present invention;

FIG. 3 is a cross section of a ceiling tile incorporating a heater fabric of the present invention;

FIGS. 4 and 5 are plan views of a heater fabric showing the indicated temperature testing positions and removed portions;

FIG. 6 is a plan view of a plaster board incorporating a heater fabric and a light fixture with the indicated temperature testing positions;

FIG. 7 is a cross section of the removed portion for the light fixture in FIG. 6; and

FIG. 8 is a plan view similar to FIG. 5 for a laminate with a heater fabric.

DESCRIPTION OF THE INVENTION The heater fabric of the present invention comprises a woven glass fabric treated with a conductive mixture of carbon and vinylidene chloride polymer to provide the desired watt density and then coated with a vinylidene chloride polymer vapor barrier having a moisture vapor transmission of 0.02 perms or less. This vapor barrier serves to protect the conducting mixture from attack by moisture and at the same time eliminates the hazard of contact with the live electric circuit.

One type of heater fabric according to the present invention is shown in FIG. 2 where the various layers are shown schematically rather than in their relative proportions. The glass fabric is coated in sequence with a conductive layer 14 of carbon and a polymer of vinylidene chloride, such as polyvinylidene chloride (PVDC) and then an outer vapor barrier layer 16 of PVDC. Copper or other suitable metal electrodes are provided to connect the voltage source to the conductive layer 14 by weaving the glass fabric with the electrodes as shown by 18 or by attaching the electrodes 20 to the coated fabric before the vapor barrier 16 is added.

A fabric softener 12 may be applied to the glass fabric 10 to impart more flexibility to the heater fabric.

The form of the carbon suitable for the conductive layer is not important for the heater fabric operation so long as the carbon particles are of a sufficient size to be embedded in a continuous coating when mixed with the vinylidene chloride polymer. Carbon black is one preferred from. Another is Vulcan XC-72 which is specifically made for use as a conducting medium and has excellent conductivity characteristics. Vulcan XC-72 has an ASTM D2516 classification of N472, an average particle size of 35 millimicrons, N surface areas of 254 square meters per gram and its DBP absorption is 200 cc/l00g. The amount of carbon in the conductive coating varies over a wide range of concentrations depending on the desired watt density of the heater fabric and coating thickness. For most practical applications, the weight ratio of carbon to vinylidene chloride polymer is in the range of 20:80 to 80:20. Carbon is calculated as 100 percent solids and vinylidene chloride polymer is calculated as 62 percent solids present in the emulsion. 1

The vapor barrier has a moisture vapor transmission of 0.02 perms or less and is formed from polymers of vinylidene chloride. Bythe terms polymers of vinylidene chloride or vinylidene chloride polymers it is meant to'include not only the homopolymer but also various high vinylidene chloride content copolymers, terpolymers, etc.

Two of the important characteristics of polymers of vinylidene chloride (Cl-l CCL are thermal stability and impermeability to gases and vapors. Various polymers of vinylidene chloride which may be employed in the present invention are known and subsequently described in Vol. 14, pages 540-579 of Encyclopedia of Polymer Science and Technology, 1971, John Wiley and Sons, lnc., which is hereby incorporated by reference.

The usefulness of polymers of vinylidene chloride depends on the permeability of the polymer which in turn depends on the density and crystallinity of the polymer. The homopolymer of vinylidene chloride has a lower impermeability than most other copolymers of vinylidene chloride and is therefore best suited for the vapor barrier. However, availability and fabrication problems arise in the use of PVDC and therefore various copolymers of vinylidene chloride may also be employed.

Polar comonomers and plasticizers increase the permeability of the polymer film as compared to PVDC and therefore use of such films depends on the requirements of the vapor barrier.

The preferred copolymers are those which comprise at least 50 molar percent of vinylidene chloride with the balance comprising another vinyl monomer such as styrene, vinyl chloride, acrylonitrile, methyl acrylate, methyl methacrylate, ethyl acrylate and vinyl acetate. Terpolymers or polymers comprising 4 or more comonomers may also be employed so long as sufficient vinylidene chloride is present to impart the necessary impermeability to the polymer film. Various graft copolymers of vinylidene chloride are also suitable for the present invention.

Commercially available vinylidene chloride polymers are Saran A (homopolymer), Saran B (vinylidene chloride-vinyl chloride copolymer), Saran C (vinylidene chloride-acrylonitrile copolymer) and Saran F (vinylidene chloride-acrylonitrile copolymer), all available from Dow Chemical.

The polymer type in the conductive coating is not as important as the vapor barrier polymer since only the high thermal stability of vinylidene chloride polymers is involved whereas the vapor barrier requires a coating.

of low permeability. Generally, however, the same type of polymers suitable for the vapor barrier are also suitable for the conductive coating with the homopolymer of vinylidene chloride being preferred for both the conductive coating and the vapor barrier.

The conductive coating thickness varies over a wide range depending on the desired use of the heater fabric and the required watt density. Generally, because of the increasing resistance of a lower limit of 0.01 OZ/Sq. yd. is maintained with an upper limit being determined by economics, overall thickness requirements and required watt densities. Therefore, a preferred range if 0.01 to 3 oz/sq. yd. and a more preferred range is 0.20 to 0.60 oz/sq. yd. It is obvious that various coating weights can be varied depending on the carbon/PVDC ratio for a given resistance. Thus, by varying the ratio of conducting carbon to polyvinylidene chloride and the coating weight,the electrical resistance can be varied over a wide range; i.e., from 5 ohms per square foot to 5,000 ohms per square foot.

The manufacture of the heater fabric of the present invention is a multi-step process which may be carried out as follows: I

l. Weave glass fabric with copper wires (electrodes) spaced at selected distances across the width of the fabric.

2. Gently heat clean the woven fabric, as for example according to the process outlined in US. Pat. 2,970,934, which is hereby incorporated by reference, to remove the residual or original starch and oil binder applied during the manufacture of the glass yarns without adversely affecting any of the copper electrodes and at the same time insuring that a clean glass surface is available on which to bond the conductive coating.

3. Coat the fabric with a formula consisting of a polymer of vinylidene chloride mixed in different ratios of carbon to give the desired electrical resistance, dry and cure this coating at temperatures of 250 to 450F for a period of /2 to 2 minutes. Drying temperatures between 250 and 450F show no effect on the electrical resistance of the glass heater fabric.

4. Apply 1, 2, or 3 coats of a vinylidene chloride poly- Single Treatment mer vapor barrier coating to obtain a moisture vapor :51:12 wmghm'zz transmission rating of 0.02 perms or less, and at the 833 ohms resistance weightl).44 oz. sq./yd. same time insulate the conducting surface against accizi w l; I dental contact with the electrical circuit. l Ohm reb'b'dnce weight As an alternate to steps 1, 2, and 3, the glass fabric could be woven in the conventional manner without the copper electrodes, heat cleaned as in step 2, and coated with the conducting coating, as in step 3. The copper electrodes would then be placed on the fabric It) by sewing or with a pressure sensitive adhesive. Following the attachment of the copper electrodes, the vapor barrier coatings would be applied to the fabric as outlined in step 4 above.

Thefollowing table is a list of the materials used in the examples with their corresponding composition.

As shown by this example, the resistance can be controlled by the coating weight when the ratio of carbon and polyvinylidene chloride is constant.

When the above samples were treated with a polyvinylidene chloride vapor barrier coating, the resistance did not change appreciably.

Example 2 Glass fabric style 7628 was heat cleaned to remove the starch and oil binder and treated as follows:

Solution A Solution B Water 900 grams Solution A 100 grams Tamol SN 45 grams Polidene M3-l20 24.0 grams TSPP l5 grams Vulcan XC-72 180 grams TABLE 1 The glass fabric was given three applications of Solu- 75 tion B and dried at 350F for 1 minute between each Material Composition m application.

T z Vulcan X072 highly reinforcing furnace. he abritc had in electrical resistance of 316 ohms ((i. l.. Cabot Inc.) carbon black per W One-half inch wide copper electrodes were sewn on Tamol SN anionic. polymeric tanning (Rohm & Haas Co.)

TSPP

Polidenc M3-12l) and dispersing agents anhydrous tetrasodium pyro hosphatc polyvinylidene chloride copolymer emulsions 0 opposite sides ofa 10 X 10 inches fabric sample and the fabric sample was then given two coats ofthe polyvinylidene chloride vapor barrier. This sample was connected to 110 volts A.C. A beaker containing 100 grams of water was placed on top of the heater fabric and the temperature of the water rose from 65to Polycryl 7-F-l acrylic polymcrs and [30F copolymers polymerized in a solvent medium and supplied in -50% solutions in toulcne. MEK and other solvents 40 Glass fabric woven glass lahric style 762% Example 3 Glass fabric style 7628 was heat cleaned to remove L the starch and oil binder and treated as follows:

Solution A Water 500 grams Solution A 100 grams Tamol SN 15 grams Polidene M3-l20 23,6 grams TSPP 5 grams Vuncan XC-72 60 grams The following examples will serve to further illustrate the present invention:

Example 1 Glass fabric style 7628 was heat cleaned to remove the oil and starch binder and then treated as follows: The followmg elecmcal reslstance was Obtamed- The fabric was treated 3 and 4 times with solution B and dried at 350F for 1 minute between each treatment. Following this the polyvinylidene chloride vapor barrier treatment was applied.

Solution A Solution Water 500 grams Solution A 100 grams Tamol SN 15 grams Polidene M3-l20 31.4 grams TSPP 5 grams Vulcan XC-72 grams The fabric was given 1, 2 and 3 applications of Solution B and dried at 350F for 1 minute between each application of Solution B. The following electrical resistance was obtained:

3 treatments 425 ohms per sq. ft.

4 treatments 316 ohms persq. ft.

By comparing examples 1, 2 and 3 at the third treatment level, the following summary can be made:

Formula Resistance Example 1 60 parts Vulcan XC-72 I02 parts Polidene M3-120 441 ohms/sq. ft.

Example 2 120 parts Vulcan XC-72 102 parts Polidene M3420 316 ohms/sq. ft

Example 3 80 parts Vulcan XC-72 102 parts Polidene M3-l20 425 ohms/sq. ft.

It has been found that there is a maximum and minimum ratio ofcarbon to polyvinylidene solution in order to make a satisfactory heater fabric in the range of 10 to 60 watts per square foot. These ratios are shown in Fig. 1. Greater or lesser ratios may be employed to achieve different power rated fabric heaters.

From the curves shown in Fig. 1, information can be extracted concerning the minimum and maximum ratios for a given resistance. For example, for a resistance of 2.500 ohms per square foot and a coating weight of 1.0 oz./sql yd. the minimum ratio of carbon to polyvinylidene solution is 40 to 60, and the maximum ratio is 58 to 42 for a fabric heater ration of 10 to 60 watts per square foot.

Example 4 Glass fabric style 7,628 was heat cleaned to remove the starch and oil binder and knife coated on. one side only with the following formula:

Water 324.0 grams Tamol SN 25.2 grams TSPP 8.4 grams Vulcan XC-72 100.8 grams Polidene M3-l20 344.0 grams The coating was dried at 300F for 1 minute.

The electrical resistance of the one-side coated fabric was 700 ohms per square foot.

The uncoated side of the above sample was then knife coated with the same solution, dried at 300F for 1. minute. Theelectrical resistance was now 360 ohms per square foot.

Example 5 A sample of heat cleaned style 7628 glass fabric was 20 treated with a 20 percent aqueous solution of Polycryl 7-F-1 and dried 1 minute at 300F. This sample was then coated as in Example number 4 with the following results:

Polycryl with l-side knife coated 860 ohms per square foot Polycryl with 2-side knife coated 315 ohms per square foot The polycry] made the finished heater fabric more flexible. Other types of fabric softeners could also be used to impart flexibility to the glass heater fabric.

Example 6 A plant trial was run on two styles of glass fabric, 1562/38 and 7628/50. The test schedule was operated as follows:

MIXING PROCEDURE:

Using a Eppenbach mixer, water, Tamol SN, and TSPP were mixed until the Tamol and TSPP dissolved.

Vulcan XC-72 was added and mixed until dispersed.

Then Polidene M-3-120 was added and mixed until a uniform mixture was obtained.

RUNNING CONDITIONS:

Run A: Coronize 50 yards of 7628/50 greige at 1250F and 20 yards per minute. Apply above formula in three dips, using submerged rolls, at 10 lbs. pad pressure. and dry between each dip at 300F. 7

Take 50 yards 7628/50 1-537 (batch oven cleaned) and 50 yards 1562/38 I-537 (batch oven cleaned) and apply above formula in three dips, using submerged rolls, at

10 lbs. pad pressure. and dry between each dip at 300F. Speed 20 yards per minute.

.Run B: r

Example 7 A representative sample of Run B from Example 6 was made with glass fabric style I562. The resulting resistance of the fabric with electrodes spaced 12 inches apart was 700 ohms per square foot.

To test the effectiveness ofthe vapor barrier, the center of the fabric (equal distance between the electrodes) was immersed in salt water and the resistance measured between the salt water and the electrodes. The resistance was found to be in excess of million ohms (5 megohms). The same sample prior to application of the vapor barrier coating had a resistance of 200 ohms.

This shows that the vapor barrier coating is protecting the conducting coating from moisture and from accidental contact with the live circuit.

To show the unexpected result of the vapor barrier coating on the uniformity of resistance and flex properties', a 1 inch wide sample of heater fabric I2 inches long was flexed over a A inch diameter rod with the following results:

Resistance of a I l2" Strip ohms/square foot Sample without Vapor Barrier Sample with Vapor Barrier Original 666 1 I65 Flexed 25 times I I64 I335 Flexed 50 times 2I64 I584 Flexed 75 times 4000 I665 Flexed I00 times 8333 I665 7: Change in Resistance I090.0 42.9

The sample without the vapor barrier showed a much larger change in resistance upon flexing than the sample with the vapor barrier coating. It is also important to note that the sample with the vapor barrier coating showed no further change in resistance after 75 flexes.

Example 8 The glass heater fabric described in this invention can be used to make a heated ceiling tile in a relatively simple manner as described in this example.

A sample of style 1562 open weave glass fabric was gether with a thickness of approximately /1 inch, having excellent acoustical properties and appearance.

When I 10 volts A.C. were connected to the electrodes. the surface of the facing fabric was warm to the touch, and after a short period of time reached a temperature of F.

To test the effectiveness of the vapor barrier coating, an area of4 square inches in the center of the panel was saturated with salt water and the resistance measured to the electrodes. The resistance was found to be 5 megohms, indicating that the vapor barrier coating was acting both as a protection against moisture and insulating the electric circuit.

Example 9 TABLE II Location Temperature F Next the same sample, as shown in Fig. 4, was cut to remove a 3 inches diameter hole in the center to produce the sample shown in Fig. 5. A voltage of l 10 volts A.C. was again connected to the sample and the surface temperature measured, as shown in Fig. 5 according to the following table. The resistance of the sample with the cutout was 260 ohms per square foot which gave a watt density of 50.8 watts per square foot.

TABLE In Location Temperature F I38 I50 I48 I30 XXX I30 I50 I52 I50 Example 10 Again using heater fabric made according to the process of Example 6, a plaster board was constructed with the heater fabric being incorporated into the wet plaster slurry. After drying, a cutout was made and a lamp fixture mounted on the board. The following table gives the temperature profile of the plaster board according to the plaster board of FIGS. 6 and 7.

11 TABLE IV TEMPERATURE PROFILE STUDY PLASTER BOARD WITH HEATER FABRIC AND NO INSULATION Board Size 16" X 32" X 3/8" The heater fabric continued to operate on a satisfactory basis.

Example 1 I Another temperature profile study was conducted on a polyester-glass laminate with the heater fabric made R 770 OHMS E I VOLTS WATTS 17.13

TABLE V Position Temperature IIO IIO 95 H0 H5 H0 I H5 H0 IIO I08 I00 I09 I08 I08 Cizss acooqomauw TABLE V-Contmued Position Temperature I6 I I0 17 I I0 18 I I0 I9 I05 20 I I0 2I I20 22 I I5 23 I20 34 I I0 25 I I0 26 I I5 27 I I0 28 I05 29 I00 30 I00 The heater fabric continued to operate on a satisfactory basis.

What is claimed is:

l. A flexible heater fabric comprising a woven glass cloth having a layer of a conductive mixture to provide a desired watt density comprising carbon and a vinylidene chloride polymer, means in contact with the conductive mixture for applying an electrical potential and a vapor barrier ofa vinylidene chloride polymer to protect the conductive mixture from moisture and accidental electrical contact.

2. A flexible heater fabric as claimed in claim 1 wherein the watt density is 10 to watts per square foot.

3. A flexible heater fabric as claimed in claim I wherein the vapor barrier has a moisture vapor transmission of 0.02 perms or less.

4. A flexible heater fabric as claimed in claim 1 wherein the means for applying an electrical potential comprise at least two electrodes.

5. A flexible heater fabric as claimed in claim 4 wherein the electrodes are copper.

6. A flexible heater fabric as claimed in claim 1 wherein the layer of the conductive mixture comprises carbon and polyvinylidene chloride and the vapor barrier consists of polyvinylidene chloride.

7. A flexible heater fabric as claimed in claim 1 wherein a fabric softener is provided between the woven glass cloth and the layer of the conductive mixture.

8. A flexible heater fabric as claimed in claim 1 wherein the layer of conductive mixture has a carbon/- vinylidene chloride polymer ratio in the range of 20:80 to :20. 

1. A FLEXIBLE HEATER FABRIC COMPRISING A WOVEM GLASS CLOTH HAVING A LAYER OF A CONDUCTIVE MIXTURE TO PROVIDE A DESIRED WATT DENSITY COMPRISING CABRON AND A VINYLDEND CHLORIDE POLYMER, MEANS IN CONTACT WITH THE CONDUCTIVE MIXTURE FOR APPLYING AN ELECTRICAL POTENTIAL AND A VAPOR BARRIER OF A VINYLIDEND CHLORIDE POLYMER TO PROTECT THE CONDUCTIVE MIXTURE FROM MOISTURE AND ACCIDENTAL ELECTRICAL CONTACT.
 2. A flexible heater fabric as claimed in claim 1 wherein the watt density is 10 to 70 watts per square foot.
 3. A flexible heater fabric as claimed in claim 1 wherein the vapor barrier has a moisture vapor transmission of 0.02 perms or less.
 4. A flexible heater fabric as claimed in claim 1 wherein the means for applying an electrical potential comprise at least two electrodes.
 5. A flexible heater fabric as claimed in claim 4 wherein the electrodes are copper.
 6. A flexible heater fabric as claimed in claim 1 wherein the layer of the conductive mixture comprises carbon and polyvinylidene chloride and the vapor barrier consists of polyvinylidene chloride.
 7. A flexible heater fabric as claimed in claim 1 wherein a fabric softener is provided between the woven glass cloth and the layer of the conductive mixture.
 8. A flexible heater fabric as claimed in claim 1 wherein the layer of conductive mixture has a carbon/vinylidene chloride polymer ratio in the range of 20:80 to 80:20. 